1. Field
One embodiment of the present invention relates to a projection optical system, exposure apparatus, and exposure method and, more particularly, to a projection optical system suitably applicable to exposure apparatus used in manufacturing microdevices, such as semiconductor devices and liquid-crystal display devices, for example, by photolithography.
2. Description of the Related Art
The photolithography for manufacturing the semiconductor devices or the like is carried out using an exposure apparatus for projecting an image of a pattern of a mask (or reticle) through a projection optical system onto a photosensitive substrate (a wafer, a glass plate, or the like coated with a photoresist) to effect exposure thereof. In the exposure apparatus, the required resolving power (resolution) of the projection optical system is becoming higher and higher with increase in integration degree of the semiconductor devices or the like.
In order to meet the demand for the resolving power of the projection optical system, it is necessary to decrease the wavelength λ of illumination light (exposure light) and to increase the image-side numerical aperture NA of the projection optical system. Specifically, the resolution of the projection optical system is expressed by k·λ/NA (k is a process factor). The image-side numerical aperture NA is represented by n·sin θ, where n is a refractive index of a medium (normally, gas such as air) between the projection optical system and the photosensitive substrate, and θ a maximum angle of incidence to the photosensitive substrate.
In this case, when one attempts to increase the image-side numerical aperture by increase in the maximum incidence angle θ, it will result in increase in incidence angles to the photosensitive substrate and in exiting angles from the projection optical system, so as to increase reflection loss on optical surfaces, and it will eventually fail to ensure a large effective image-side numerical aperture. Therefore, a liquid immersion technique has been proposed in International Publication WO2004/019128, which increases in the image-side numerical aperture by filling the optical path between the projection optical system and the photosensitive substrate with a medium like a liquid having a high refractive index.